Due to it's mixed ionic and electronic conductivity (MIEC), rare earth doped ceria (REDC) represents a promising class of materials for electrochemical devices like gas separation membranes, solid oxide fuel cells (SOFCs) and solid oxide electrolysis cells (SOECs). Sintering of rare earth doped ceria (REDC) is usually done in air at high temperatures, in the range of 1400° C to 1600° C, and dwelling time of several hours to achieve almost theoretical density. A promising approach to improve densification at lowered temperatures is field assisted sintering/spark plasma sintering (FAST/SPS) in combination with low oxygen partial pressures followed by controlled re-oxidation at high temperature.
In this work, Ce0.9Gd0.1O1.95-δ (i.e. GDC10, gadolinium-doped ceria, with Gd 10 mol. %) has been used for conducting a related sintering study. Ceria-based materials (e.g. GDC) show mechanical stresses due to chemical expansion in low oxygen partial pressure, p(O2), because of the large concentration of oxygen vacancies and changes of the valence state of Ce from (Ce4+, 0.97 Å) to (Ce3+, 1.14 Å). Therefore, sintering of the GDC material at low p(O2) followed by uncontrolled re-oxidation easily results in mechanical failure of the sample due to crack formation. The goal of this work is to take advantage of sintering GDC material by FAST/ SPS under low oxygen partial pressure while avoiding cracking of the samples by re-oxidizing the sample at elevated temperature. Several challenges needed to be overcome like choice of the right tool material, controlled change of atmosphere at the elevated temperature and others.
Sintering experiments were conducted in different sintering atmosphere in FAST/ SPS and conventional furnaces. The densification behavior was studied in different sintering atmospheres. Finite element (FE) simulations of the sintering processes were carried out using the parameters of real sintering cycles. In this work, the well-known constitutive equations for linear viscous materials (Skorohod-Olevsky viscous sintering (SOVS) model) were modified with the inclusion of measured uniaxial viscosities in different atmospheres, which are then implemented in ABAQUS as a user-defined subroutine to describe the densification behaviors of the GDC and to calculate the grain growth during sintering. The numerical work is concluded by the comparison of the experimental relative densities, shrinkages and grain growth with the simulated values.